1. Field of the Invention
The present invention generally relates to an all optical analog-to-digital converter, and more particularly to such an apparatus that employs an optical interferometer that uses two optical signals that differ in wavelength by a factor of two.
2. Description of the Prior Art
It is often desirable to convert an analog amplitude varying signal to a digital set of values which corresponds to various voltages in the analog waveform to generate a corresponding digital signal. Conventional approaches generally rely on iterative and/or comparative techniques for determining a digital signal based on an analog waveform voltage. In particular, a common conventional approach compares the actual voltage of the analog amplitude varying signal to a comparison voltage which is generated from a digital word. Various digital words are utilized to create comparison voltages which are then rapidly compared to the actual voltage to determine whether the comparison voltages are greater or less than, in an instant of time, the analog amplitude varying signal. Through a continuous iterative comparison process, a digital word which corresponds to the actual voltage of the analog amplitude varying signal is generated. The digital word is recorded for that instant of time and the same iterative and/or comparative process is repeated for subsequent instants of time corresponding to the analog signal. This conventional approach is limited in speed by the iterative comparison process.
An approach for performing analog-to-digital conversion that eliminates this time consuming iterative comparison process is described in U.S. patent application Ser. No. 09/089,844, filed Jun. 17, 1998, entitled "All Optical Analog-To-Digital Converter", and assigned to the same Assignee as the present invention. In this approach, an analog signal is converted to an optical signal that varies in wavelength in accordance with the amplitude of the analog signal. This optical signal is then analyzed in parallel by an array of optical interferometers that have optical path length differences that correspond to the weighting factor of each digital bit in the digital word that is used to represent the analog voltage at a particular instant in time. The individual optical interferometers simultaneously deliver, via interference the appropriate optical output from each of their two outputs so that the optical levels, when detected by photodetection, will generate, in parallel, the appropriate digital bits that make up the digital word that corresponds to the analog voltage at that particular instant in time.
The process for creating the appropriate state of interference in each of the optical interferometers requires that the optical path length difference in each optical interferometer be maintained to within a fraction of a wavelength of the light being interfered. Optical interferometers made using optical fiber or silica waveguide are not stable devices. They are particularly susceptible to uncontrollable conditions, such as temperature variations. As the temperature proximate the optical interferometer changes, the path length of the optical fibers or silica waveguide making up its legs likewise change. This results in a change in the interference pattern created by the optical interferometer. To compensate, the optical interferometer must be tuned continuously.
An apparatus and method for tuning an optical interferometer is known in the art. An example of such an apparatus is described in an article by Eric A. Swanson, Jeffrey C. Livas and Roy S. Bondurant, entitled "High Sensitivity Optically Preamplified Direct Detection DPSK Receiver With Active Delay-Line Stabilization," in IEEE Photonics Technology Letters, Vol. 6, No. 2, February 1994. This article describes an optical communication system that modulates digital information onto transmitted light using differential phase shift keying (DPSK) and then demodulates this information using an actively tuned unbalanced Mach-Zehnder optical interferometer that is tuned using an apparatus and a method known in the art. The unbalanced Mach-Zehnder optical interferometer has an additional optical path length in one leg that provides a propagation delay duration of one data bit. The imbalance in the Mach-Zehnder optical interferometer enables light in one data bit to be optically interfered with light in the data bit immediately following this data bit. The relative state of optical phase between these two DPSK data bits determines in which of the two output legs of the interferometer light is produced provided that the unbalanced Mach-Zehnder optical interferometer is properly tuned within a fraction of a wavelength of the light. Light produced from one leg constitutes digital "ones" while light produced in the other leg constitutes digital "zeros" in the transmitted digital information signal.
The apparatus described in the article includes a laser and a phase modulator for producing an optical DPSK signal at a preselected wavelength, a tunable unbalanced Mach-Zehnder optical interferometer, a dual balanced detector and a feedback electronic circuit coupling the signal developed across one detector of the balanced detector to one leg of the Mach-Zehnder interferometer. Two different approaches are described for tuning the optical path length in the unbalanced Mach-Zehnder optical interferometer. In the first approach, the interferometer is made of optical fiber and one leg of the interferometer is wrapped around a piezoelectric transducer (PZT) that enables an electronic signal to stretch the fiber, thereby increasing the optical path length. In the second approach, the interferometer comprises a silica integrated optical waveguide with an integral thermal heater that enables an electronic signal to increase the temperature of one leg of the interferometer, thereby increasing the optical path length. To tune the Mach-Zehnder interferometer a small electronic dither signal is applied to the actively tuned optical path length to provide a feedback signal for the electronic controller. This enables proper adjustment of the optical path length. Electronic synchronous detection techniques on this dither signal are used to provide the appropriate corrections to the optical path length, enabling the error in tuning to be below an acceptable level. This same tuning approach can be utilized to tune the optical interferometer in the all optical analog-to-digital converter, but there are some adverse consequences.
The prior art approaches for actively tuning an optical interferometer have several disadvantages. First, they introduce an undesired optical intensity dither on top of the original optical signal that is intended to be extracted. This dither arising from the intentional dither of the optical path length is actually a source of noise that degrades the fidelity of the original signal. In the case of the all optical analog-to-digital converter, this dither leads to significant errors in the determination of the digital word since each data bit in this word can be affected by the dither. Second, the approach using the heater to perform the dither and tuning is restricted to relatively low frequencies of dither due to the relatively large thermal time constant of the heater. Third, the approaches introduce a small dithering variation in the interference state delivered at the output of the Mach-Zehnder interferometer. This precludes the use of the interferometer in applications where an absolute quiet state of interference must be maintained such as is the case in the all optical analog-to-digital converter.
What is needed, therefore, is an improved all optical analog-to-digital converter that employs an improved optical interferometer tuning approach which utilizes an additional optical signal for tuning without introducing any dither in its optical path length.